Integrating Diffraction, Advanced Microscopy and NMR Methods to Explore NOM-Mineral-Cation Interactions 

Tuesday, 16 December 2014
R James Kirkpatrick1, Geoffrey Bowers2, Brennan Ferguson2, Haley Argersinger2, Uddigiri Venkateswara3, Bruce Arey4 and Mark Bowden4, (1)Michigan State University, College of Natural Science, East Lansing, MI, United States, (2)Alfred University, Chemistry, Alfred, NY, United States, (3)Michigan State University, Chemistry, East Lansing, MI, United States, (4)Pacific Northwest National Laboratory, William R. Wiley Environmental and Molecular Sciences Laboratory, Richland, WA, United States
Combined 43Ca nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and helium ion microscopy (HeIM) results provide novel insight into the interactions among NOM mineral (smectite) surfaces, dissolved ions, and water; and the effect of hydration state and pH on these interactions.  The molecular-scale 43Ca NMR results for a Ca-smectite-Suwannee River NOM-H2O system suggests that Ca2+ in smectite-NOM composites  behaves more like Ca2+ in smectites without NOM than Ca2+ in NOM alone also support the idea that much of the Ca2+ in the composites bridges between the NOM and the mineral surface. The NMR results also show that the NOM protonation state (pH) during composite synthesis has little effect on the local molecular-scale coordination environment and dynamics of Ca2+ and that H2O activity is the most important control of composite basal spacing and ion dynamics. XRD results corroborate the formation of smectite-NOM composites and suggest significant cation-NOM association. With NOM present, there is loss of the (005) smectite reflection; no evidence of amorphous NOM; and growth of a broad peak near 3.5 Å, similar to the spacing observed in the benzene ring stacking of graphene. The XRD results also show that the composite materials expand upon exposure to H2O, suggesting that H2O has access to the interlayer spaces and cations. HeIM images offer an explanation as to why there is no pH-dependence in the Ca2+ molecular-scale behavior. Ca-smectite-NOM composites formed at pH 12 appear to be a relatively homogeneous on the <~100 nm scale, whereas composites formed by rapidly decreasing the pH from 12 to 2 show both composites and separate NOM aggregates with little porosity at scales >10 nm. It is likely the formation of NOM aggregates is dominated by hydrophobic interactions that exclude Ca2+ and thus that 43Ca NMR observes bridging Ca2+ similar to that in the pH 12 samples.